Chris has a master's degree in history and teaches at the University of Northern Colorado.

Our immune systems are pretty good, but sometimes pathogens can sneak in and infect living cells. Luckily, we've got a back up system. Let's explore cell-mediated response and see how this protects our bodies from infections that may otherwise go untreated.

Cell-Mediated Immune Response

Your body is a war zone, constantly fighting off diseases and infections and all sorts of harmful things. Luckily, we've got some pretty good defense systems. Now, generally, when a pathogen enters the body, it's targeted by white blood cells, in response to antibodies, which we call humoral immunity. The only issue is that if the pathogen manages to actually infect a living cell within the body, the antibodies are ineffective. The body won't recognize a pathogen within a living cell and the humoral immune response passes it by. But luckily we've got a backup system.

The cell-mediated immune response identifies and destroys infected cells, preventing the bacteria or virus from spreading any further. Cell-mediated immunity provides a double layer of security, keeping us safe in the continual battle of existence.

T Cell Activation

Normal white blood cells, often called b cells, need antibodies to help them identify pathogens, which is why they can't find pathogens hiding inside an infected cell. This means that cell mediated immunity relies on a different cast of characters. These are special white blood cells that target pathogens within a cell, called T lymphocytes, or T cells for short. The T cells are activated in response to specific antigens, substances indicating the need for an immune response that are created by the infected cell.

The process begins with what is called an antigen-presenting cell, a kind of white blood cell that produces antigens. This APC eats a pathogen, digests it, and creates hundreds of antigens to help the immune system recognize the intruding disease. The APC presents these antigens on its surface using a protein called MHCII. The antigen on the MHCII alerts T cells that there is a pathogen within the body and tells them what they are looking for. T cells are activated when they receive these instructions from APCs or infected cells via a molecule on the T cell surface responsible for recognizing MHC antigens, called the T cell receptor.

So, quick recap. Pathogens enter the body; APC creates antigens, which they display with the protein MHCII; T cells recognize this antigen using the T cell receptor and set off to find and destroy the pathogen.

Fighting Pathogens

Now that the T cell knows what it's looking for, it is able to identify the pathogen hiding within existing cells. Now, finding and eradicating the infection actually requires a few different kinds of T cells, each with a distinct focus. The two main types are cytotoxic T cells and helper T cells.

Cytotoxic T cells are the main cells responsible for cell-mediated immunity that identify pathogens and destroy the host cell. That's what they do, they destroy the infected cell to prevent the infection from spreading. Once a cytotoxic T cell is activated it will immediately begin cloning itself at a rapid rate, creating dozens of T cells already activated and programed to identify the intruding pathogen. Some of these T cells attack the infected cell, initiating apoptosis, or programmed cell death. It's kind of like an automatic self-destruct sequence that's encoded within the cell's DNA. The rest of the cytotoxic T cells don't attack anything. They just serve as memory cells, waiting to continue cloning themselves, should the infection ever return.

The other kind of T cell is the helper T cell, which releases a signal molecule to recruit other immune response cells. This signal molecule, called a cytokine helps the regular immune systems to identify pathogens within a cell. Some of the cells attracted by cytokines are the natural killer cells, types of white blood cells that can also destroy infected cells when activated and phagocytes, which kill infected cells by engulfing them. Essentially, one poisons the infected cell, and the other eats it. Helper T cells therefore make infected cells visible to more than just the T cells, bringing the entire force of the immune system together. Now that's a battle strategy that works!

Lesson Summary

When pathogens enter the body and infect cells, they cannot be detected by the humoral immune response, in which white blood cells use antibodies to target dangerous substances. So the immune system uses a special group of white blood cells that can identify pathogens within a cell called T lymphocytes or T cells.

This is called the cell-mediated immune response, in which T cells identify and destroy infected cells. Rather than killing the pathogen alone, this system requires the destruction of the infected cell so that the infection does not continue to spread. T cells are activated when an antigen-presenting cell, a kind of white blood cell that produces antigens, creates antigens and presents them using a protein called MHCII. The T cell uses the antigen to identify infected cells.

There are two main types of T cells at work here. Cytotoxic T cells destroy infected cells by initiating apoptosis, or program cell death. Helper T cells release a signal molecule called a cytokine that recruits other immunity response cells by helping them identify the infected cells. So, when you're sick, just skip the A-team and go straight for the T team.

Summary:

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